Image sensors are semiconductor devices for converting an optical image into an electric signal. In general, an image sensors is either a charge coupled devices (CCD) or a CMOS (Complementary Metal Oxide Silicon) image sensor.
The charge coupled device (CCD) is provided with a matrix of photo diodes (PD). Each photo diode converts an optical signal into an electric signal. The CCD also includes a plurality of vertical charge coupled devices (VCCD). Each of the VCCDs is formed between vertical lines of the photo diodes in the matrix for transmission of charges generated at the photo diodes in a vertical direction. The CCD also includes a horizontal charge coupled device (HCCD) for transmission of the charges transmitted through the VCCDs in a horizontal direction. In addition, the CCD includes a sense amplifier for sensing the charges transmitted in the horizontal direction and outputting an electric signal.
However, the CCD is disadvantageous in that it has a complicated driving method, exhibits high power consumption, and is produced via a complicate fabrication process involving multiple photo process stages. Moreover, CCDs have another disadvantage in that it is difficult to include a CCD in a small product due to the difficulty in integrating a control circuit, a signal processing circuit, an A/D converter, and the like on a CCD chip.
Recently, the CMOS sensor has been heralded as the next generation image sensor that can overcome the disadvantages of CCDs. The CMOS image sensor is a device that employs CMOS technology to capture an image. Specifically, a control circuit, a signal processing circuit, and the like are used as peripheral circuits for successively detecting outputs from pixels using MOS transistors. A MOS transistor is formed on the semiconductor substrate for each pixel. That is, the CMOS image sensor has a photo diode and a MOS transistor formed within each unit pixel. By monitoring the switching of the MOS transistors, the CMOS image sensor successively detects electric signals from the photo diodes of the unit pixels to reproduce an image.
The CMOS image sensor exhibits low power consumption and enjoys a simple fabrication process as a result of fewer photo process stages. Moreover, the CMOS image sensor is advantageous in that products incorporating the CMOS image sensor can easily be made smaller because a control circuit, a signal processing circuit, an A/D converter, and the like can be integrated on a CMOS image sensor chip. Accordingly, the CMOS image sensor has a wide range of applications, such as in a digital still camera, in digital video cameras, and the like.
A prior art CMOS image sensor will now be described with reference to the attached drawings. FIG. 1 illustrates a circuit equivalent to one pixel of a prior art CMOS image sensor. FIG. 2 is a cross-sectional view of the prior art CMOS image sensor.
Referring to FIG. 1, the pixel unit of the prior art CMOS image sensor is provided with one photo diode (PD), and three NMOS transistors T1, T2, and T3. The photo diode PD has a cathode connected to both a drain of the first NMOS transistor T1 and a gate of the second NMOS transistor T2. Both the first and the second NMOS transistors T1, T2 have sources connected to a power source providing a reference voltage VR. The first NMOS transistor T1 has a gate connected to a reset line providing a reset signal RST. The third NMOS transistor T3 has a source connected to the drain of the second NMOS transistor. It also has a drain connected to a reading circuit (not shown) via a signal line, and a gate connected to a row selection line providing a selection signal SLCT. The first NMOS transistor T1 is called a reset transistor, the second NMOS transistor T2 is called a drive transistor, and the third NMOS transistor T3 is called a selection transistor.
The greater the light reception of the photo diode PD, the better the photosensitivity of the image sensor. Consequently, there have been efforts to increase the ratio of an area of the photo diode to the entire area of the image sensor, (i.e., to increase a fill factor), to thereby enhance the photosensitivity of the image sensor. However, since elimination of a logic circuit having transistors and the like from the CMOS image sensor is basically impossible, efforts to increase the fill factor have been inherently limited, because the fill factor may only be increased within a limited area.
Faced with such a problem, focusing technology has been suggested for enhancing the photosensitivity of the photo diode. In such approaches, the paths of light rays incident on regions other than the photo diode PD are changed, so that the light rays are focused onto the photo diode PD. A common approach to re-focusing light rays in this manner is microlens forming technology.
Referring to FIG. 2, a prior art CMOS image sensor having a microlens is shown. The image sensor of FIG. 2 is provided with a semiconductor substrate 101 having one or more active region(s) defined by one or more device isolating film(s) 102, one or more photo diode(s) 103 formed on predetermined portion(s) of the active region(s), and an interlayer insulating film 104 located on the surface of the device isolating film(s) 102 and on the surface of the photo diode(s) 103. Although not shown in FIG. 2, the interlayer insulating film 104 includes light shielding layer(s) at predetermined location(s) to prevent light from reaching regions other than the photo diode region(s) 103.
Color filter layer(s) 105 for transmitting light of a particular wavelength to corresponding ones of the photo diode(s) 103 are located on the interlayer insulating film 104 over their respective photo diode(s) 103. An over coat layer 106 is located on the interlayer insulating film 104 and on the color filter layer(s) 105. One or more microlens 107 for focusing light are located on the over coat layer 106 over the photo diode(s) 103.
Each microlens 107 refracts light running parallel to a light axis of the microlens 107 to a focal point on the light axis. Since each image sensor has tens of thousands of microlenses 107, the microlenses 107 must provide the same effect to produce a clear image. Thus, the performances of the microlenses 107 are so important that the quality of the CMOS image sensor is dependent thereon.
A method for fabricating the microlenses 107 of the prior art CMOS image sensor will now be described. FIGS. 3A to 3D are cross-sectional views illustrating a prior art microlens at various stages of fabrication.
As described with reference to FIG. 2, an over coat layer 106 is formed on the entire surface of the interlayer insulating film 104 and the color filter layer(s) 105 formed thereon. Then, as shown in FIG. 3A, a photoresist film 107 is coated on the over coat layer 106.
Referring to FIG. 3B, the photoresist film 107 is selectively patterned by photolithography to form a plurality of microlens unit patterns on the over coat layer 107. The microlens unit patterns are located over the photo diodes 103. Referring to FIG. 3C, the photoresist film 107 is baked at a temperature of about 150° C. to melt the microlens unit patterns to form convex microlenses 107a. 
The above process of fabricating microlenses of a CMOS image sensor is simplified by the fact that the microlens is formed of a material which is identical to the photoresist film. However, the prior art CMOS image sensor and method for fabricating the same have certain disadvantages.
For example, it is necessary to form the microlenses 107 in a last phase of the fabrication process because the microlens is formed of the photoresist material, which has a low melting point. As a result, the color filter layer(s) 105, the interlayer insulating film 104, and the over coat layer 106 separate the microlenses 107 from their respective photo diodes 103 by a significant distance, thereby resulting in poor focusing power of the microlenses 107.
Further, it is difficult to control the radius of curvature of the microlenses 107, and to form uniform microlenses 107 on the whole, because the microlenses 107 are formed by baking the photoresist pattern.
Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts.